Measuring geometric distortion due to gradient
non-uniformity has previously required image processing tools specific to the
phantom design. Here we demonstrate geometric distortion measurement achieving
a precision of 0.3mm over 95% of the useful volume (6.2 litres) of a phantom
using off-the-shelf non-linear registration. This method can be used with any
sufficiently detailed phantom. Geometric distortion is measured in a short-bore
interventional MRI system, finding residual errors of 2mm at a distance of 9cm
from the isocenter after the built-in gradient non-uniformity correction.